Can't restore windows after sudden total minimization

[yotama9]

Hi guys. I hope the title covers my problem properly.

Occasionally, there is something strange that happens on my system. All the windows are suddenly minimized, and I can't restore any of them. Clicking on on the window in the taksbar momentarily shift the focus to the clicked one, but then the focus is shift to another one. If I all-tab or use the show-all-windows shortcut I see all of them, but again, I can't restore any of them. I think this happen after I hit some keystroke, but I don't know which one, and I can't restore this intentionally. The only way I've managed to overcome this is by restarting mdm.

Did anyone else encounter this? How can I overcome this?

Thanks.